Erythrocyte (Na ion plus K ion) ATPase activity has been measured in Caucasian and Pima Indian subjects. Obese Pima subjects have a significantly reduced level of enzyme activity compared to normal or obese Caucasians. Additional studies will determine the importance of this lesion as a genetic factor which promotes obesity. For this purpose, the activity will also be measured in diploid fibroblasts from thin and obese Pima Indian and Caucasian subjects.